Novel UV curable polyacrylate/solvent compositions

ABSTRACT

There is provided a novel substantially anhydrous UV curable composition comprising a monomer and a solvent wherein the monomer is a multi-functional non-oxygen inhibited polyacrylate and the solvent is a low volatility solvent containing at least 2 oxygenated functions which is substantially incorporated into the polymer during the curing step. There is also provided a method of curing such compositions which avoids the conventional drying step required heretofore.

RELATED APPLICATIONS

This application is a continuation in part of applicant's copendingapplication Ser. No. 818 244 filed Jan. 13, 1986.

BACKGROUND OF THE INVENTION

UV curable coatings have been, in the main, composed of three (3)components--(1) a UV activated catalyst, (2) an unsaturated polyester ofa multi-functional acrylate or acrylate oligomer and, (3) a lowviscosity monomer such as styrene, an acrylate, methacrylate, vinylpyrrolidone, or vinyl caprolactone, etc. A newer type of UV curablecoating replace group three (3) by solvents which are removed prior toUV curing.

It would be desirable to avoid the toxicity problem of the thirdcomponent of the coating while avoiding and also the need for solventremoval.

Multi-functional, non-oxygen inhibited polyacrylates, such asdipentaerythritol hydroxy pentacrylate are a well known group of UVcurable monomers. These are however too viscous to be readily used inmany coating procedures where thin coatings (final after-cure thickness)of between 3 to 4 microns, up to about 250 microns (1 mil) are required.Traditionally, as stated above, this problem had been overcome by usingvinyl monomers or heat dried solvents. Many of the previously usedsolvent/diluents have the disadvantage of being environmentallyhazardous. This manifests itself either in the formation of hazardousvapor during the curing step when the solvent is driven off or else thesolvent/monomer mixture gives off undesirable vapors or is a skinirritant against which protective clothing would be required.Unfortunately however, in many locations where these materials are used,ventilation is either inadequate or the proper protective clothing isnot used. It would therefore be desirable to find a solvent system whichwould avoid either or both of these problems.

SUMMARY OF THE INVENTION

It has been the surprising finding of the present invention that whencertain well known solvents, all carrying at least two oxygen containingfunctions which are more concisely defined hereinbelow, are utilized assolvents for the multi-functional, non-oxygen inhibited polyacrylates,monomers in the range of between 9 and 1 part of the monomer per part ofsolvent, the solvent is, for the most part, apparently incorporated intothe polymer system and is not driven off as a vapor which must behandled in the conventional manner. The UV curable compositions of thepresent invention are substantially anhydrous.

Furthermore, certain solvents in this category are substantiallyinnocuous and are not skin irritants when they come into contact withhuman skin. The solvents utilizable in the present invention can begenerally expressed within the following formula: ##STR1## wherein R₁ isa substituted straight or branched chain alkanetriyl moiety of 2 to 6carbon atoms,

R₂ is selected from the group consisting of hydrogen, hydroxy and COOH,

R₃ and R₄ are selected from the group consisting of hydrogen, OH,--(CO)OR₅ --, O(CO)R₆, R₁ O and R₇ OR₈ O--,

wherein R₅, R₆, R₇ and R₈ are straight or branched chain alkyl orhydroxyl alkyl or 1 to 5 carbon atoms and may be the same or differentprovided that where R₂ is hydrogen or hydroxy, R₃ and R₄ are other thanhydrogen or other than simultaneously hydroxy,

further provided that where R₂ is linked to R₃ or R₄, said linkage is alactone linkage formed from the corresponding compound wherein R₂ isCOOH and R₃ or R₄ are hydroxy.

Expressed in more general terms, the solvents include lower alkanoicacids (some of which are respiratory and skin irritants); the lactonesof lower alkanoic acids, bisalkoxy substituted alkyl ethers, thealkanoyl and dialkanoyl esters of diols and hydroxy alkanoic esters andhydroxy ethers of lower alkanols or alkadiols.

The compositions of the present invention have the particular advantagethat they may be cured directly. The conventional intermediate dryingstep prior to exposure to the source of UV light used to cure them isnot required.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The UV curable compositions of the present invention comprise amulti-functional, substantially non-oxygen inhibited polyacrylate, asolvent/diluent as well as a catalyst and a plasticizer. The use ofcatalysts and plasticizers is substantially conventional. The mostsuitable multi-functional polyacrylates utilized as monomers comprisethose incorporating pentaerythritol. For purposes of exemplification butnot limitation, there may be mentioned pentaerythritol triacrylatepentaerythritol tetracrylate modified by reaction with aromatic oraliphatic isocyanates such as Mobay aromatic isocyanate Desmondur IL andaliphatic isocyanate Desmondur N75 and dipentaerythritolhydroxypentacrylate, the later being especially preferred. Thesepolyacrylates which are excellent and fast curing monomers, aregenerally speaking, too viscous to be readily utilized in thin coatingssuch as those required for covering labels and the like. The viscosityis cut by using a solvent/diluent. Utilizing the solvent/diluents of thepresent invention in a ratio of between 1 part of solvent to between 1and 9 parts of monomer, the viscosity can be substantially cut. Forexample, using a ratio of 2 parts of monomer (DPHPA) to 1 part ofsolvent (dipropylene glycol) the viscosity is reduced from approximately4,400 cps at 26°/C. to approximately 400 cps at the same temperature.

It is further preferred that the solvents of the present invention havean elevated boiling point, suitably they should have a boiling pointexceeding 115° C.

The entire composition should be substantially anhydrous. The efficiencyof cure is reduced with increasing water content of the compositionthus, it is desirable to maintain the water content of the entirecomposition below 2% preferably below 1% by weight. Water is notincorporated into the cured matrix and separates, causing a poor porousfilm, whose mechanical properties are inferior.

The solvents of the present invention may be generally expressed withinthe following formula: ##STR2## R₁ is suitably a straight or branchedchain alkanetriyl moiety of 3 to 6 carbon atoms, for example,ethanetriyl, propanetriyl, butanetriyl, pentanetriyl or hexanetriyl,most preferably propanetriyl.

R₂ is suitably hydrogen, hydroxy or COOH.

R₃ and R₄ are selected from the group consisting of hydrogen, hydroxyl,alkoxycarbonyl, alkylcarboxy and alkoxy and alkoxy-alkoxy groups whereinthe alkyl moiety may be further substituted by a hydroxy group.

While the alkyl groups may have between 1 and 5 carbon atoms, it isgenerally preferred that the alkyl moiety be ethyl, methyl or propyl. Itshould be noted for example, that acetic acid is an excellentsolvent/diluent form the point of view of providing a fast hard cure.However, due to its irritant properties, although it is operative, itwould probably not be used in commercial practice.

While the compositions of this invention give satisfactory results witha wide variety of solvents, excellent results are obtained where thesolvent has a skeleton of 3 carbon atoms. Included in this group are theesters of lactic acid, especially ethyl lactate, dipropylene glycol (di2-hydroxypropyl ether), propylene glycol monomethyl ether, and propyleneglycol monomethyl ether acetate. It is to be noted that the boilingpoint range of these especially preferred solvents lies between 120° C.and 233° C.

As stated previously, the other components of the curable compositionare conventional. As a catalyst, there may be utilized 1-hydroxycycloethylphenyl ketone (sold by Ciba as Irgacure 184), utilized in a ratioof about 1 part of catalyst to between 10 and 50, suitably 25 parts ofmonomer and, as a plasticizer, an epoxy resin such as Epirez 510 (soldby Celanese) which is utilized at a ratio of about 3 parts ofplasticizer per 10 to 50, suitably 25 parts of monomer. All parts beingby weight.

The curable compositions of the present invention may be utilized in thefollowing, substantially conventional manner. An amount ofsolvent/diluent is added to the monomer which is sufficient to reduceits viscosity at about 26° C. to between about 500 to about 20, suitablyto about 200 cps.

The material to be coated by the composition is then coated by anyconventional coating means, such as printing, screening, spraying,drawing down, and the like, to provide a coating which, upon curing,provides a layer of thickness between from about 3 to about 4 microns upto about 250 microns. The substrates upon which these coatingcompositions are placed include paper, glass, vinyl, polycarbonate, SBSboard, polyester, and the like. This list is given for purposes ofexemplification and not limitation.

The coating is then exposed to ultraviolet light in the conventionalmanner, however the previously conventional intermediate drying stepprior to the cure step is not needed and is omitted. For coatings of theforegoing thickness, it has been found that exposure to a 200 watt UVlamp (American Ultraviolet Portacure Unit) at a distance ofapproximately 10 centimeters from the coating for less than one (1)second is sufficient to obtain a hard, substantially scratch-proofsurface.

EXAMPLE I

An ultraviolet curable composition is prepared utilizing the followingcomponents:

    ______________________________________                                        dipentaerythritol hydroxypentacrylate                                                                  250    g.                                            propyleneglycol monomethylether                                                                        65     g.                                            Irgacure 184             10     g.                                            Epirez 510               30     g.                                            ______________________________________                                    

The foregoing components are mixed to provide a coating composition ofapproximately 200 cps at 26° C. A bar is wrapped with No. 3 gauge wireplaced in the coating solution and drawn over a vinyl sheet substrate.The coating on the substrate is then exposed for less than one (1)second, approximately 10 centimeters from a 200 watt ultraviolet lamp(American Ultraviolet Portacure Unit). There is provided a clear hardcoating meeting standard requirements for scratch resistance.

EXAMPLE II

    ______________________________________                                               DPHPA          10     g.                                                      Solvent        5      g.                                                      Irgacure 184   0.75   g.                                               ______________________________________                                    

In accordance with the foregoing procedure, tests were carried out with22 different solvents as set forth in Table 1 below. The results ofthese are indicated in the Table. The designation Excellent means that asatisfactory cure was obtained in less than one (1) second, whereas thedesignation Good indicates that a satisfactory cure was obtained but anexposure in excess of one (1) second was required. An Asterisk (*)indicates a solvent/diluent outside the scope of the Claims of thepresent invention.

                  TABLE 1                                                         ______________________________________                                        Diluents and Plasticizers Tested                                              [Ratings on Cure Speed: Excellent, Good, Poor, No Good,                       Incompatible]                                                                 ______________________________________                                        N-propyl acetate        No Good*                                              Ethylene glycol diacetate                                                                             Good                                                  Bis-(2-Methoxyethyl) ether                                                                            Good                                                  Glycerin                Incompatible*                                         Ethyl Lactate - CH.sub.3 CHOHCOOC.sub.2 H.sub.5 (b.p.                                                 Excellent                                             154° C.)                                                               Dipropylene Glycol - (CH.sub.3 CHOHCH.sub.2).sub.2 O                                                  Excellent                                             (b.p. 233° C.)                                                         Dow DE-Diethylene glycol ethyl ether                                                                  Good                                                  DOP (Dioctylphthalate)  No Good*                                              Propylene glycol        Incompatible*                                         Dipropylene glycol monomethylether                                                                    Good                                                  Butyl Di glyme          Poor*                                                 Propylene glycol monomethylether                                                                      Excellent                                             CH.sub.3 OCH.sub.2 CHOHCH.sub.3 (b.p. 120°)                            Octanol                 No Good*                                              DBE DuPont Mixed Aliphatic Dimethyl                                                                   Poor*                                                 Esters                                                                        Tripropyleneglycol monomethyl ether                                                                   Poor*                                                 Butyl lactate           Good                                                  Butyrolactone           Good                                                  Propylene glycol monomethylether                                                                      Excellent                                             acetate                                                                       Diacetone alcohol       Poor*                                                 2-Ethyl-1-hexanol       Incompatible*                                         ______________________________________                                    

RESULTS

Ethyl lactate has the most desirable properties for a diluent; however,propyleneglycol monomethylether acetate, dipropyleneglycol, propyleneglycol monomethylether also highly desirable.

I claim:
 1. A method of curing a substantially anhydrous UV curable composition of less than 2% w/w water consisting essentially of between 1 and 9 parts of monomer and 1 part of solvent of a b.p. exceeding 115° C. wherein the monomer is a multi-functional, non-oxygen inhibited polyacrylate and the solvent is a member selected from the group of solvents of the formula ##STR3## wherein R₁ is an straight or branched chain alkanetriyl moiety of 3 to 6 carbon atoms, R₂ is selected from the group consisting of hydrogen, hydroxy and COOH,R₃ and R₄ are selected from the group consisting of hydrogen, OH, --(CO)OR₅,--O(CO)R₆, R₇ O-and R₇ OR₈ O--, wherein R₅, R₆, R₇ and R₈ are straight or branched chain alkyl or hydroxyl alkyl of 1 to 5 carbon atoms and may be the same or different provided that where R₂ is hydrogen or hydroxy, R₃ and R₄ are other than hydrogen or other than simultaneously hydroxy, provided that where R₂ is hydroxy only one of R₃ and R₄ may be hydroxy, further provided that where R₂ is linked to R₃ or R₄, said linkage is a lactone linkage formed from the corresponding compound wherein R₂ is COOH and R₃ and R₄ are hydroxy, in the presence of an UV activated catalyst, which comprises exposing said composition in the undried state of a source of UV radiation.
 2. A method of claim 1 wherein the composition additionally comprises a plasticizer.
 3. A method of claim 1 wherein there are utilized 1 part by weight of catalyst per 10 to 50 parts by weight of monomer.
 4. A method of claim 1 utilizing a composition comprising between 1 and 2 parts of a monomer per part of solvent.
 5. A method of claim 10 wherein R₂ is COOH and R₃ and R₄ are hydrogen.
 6. A method of claim 1 wherein R₂ is hydrogen, R₃ is hydroxy and R₄ is--(CO)OR₅.
 7. A method according to claim 6 wherein R₁ is ethanetriyl, and wherein R₅ is ethyl or butyl.
 8. A method in accordance with claim 7 wherein the solvent is selected from the group consisting of ethyl lactate and butyl lactate.
 9. A method according to claim 1 wherein the solvent is dipropylene glycol.
 10. A method according to claim 1 wherein the solvent is propylene glycol monomethyl ether.
 11. A method according to claim 1 wherein the solvent is propylene glycol monomethyl ether acetate. 